The term “microresonator,” as used herein, refers to a mechanical or electromechanical resonator including a resonant element fabricated on a microscale, i.e., on a scale of micrometers to millimeters. Microresonators have important applications in various fields, including signal processing and sensing. For example, microresonators fabricated from aluminum nitride (AlN) are used in radiofrequency (RF) filters, accelerometers, and sensors. AlN is one example of a piezoelectric material that responds both electrically and mechanically to applied electric signals, and that can be formed into an electroacoustic resonator capable of modifying such signals.
Microscale fabrication techniques are sufficiently developed to afford control over geometrical properties of the resonant element such as its vertical thickness, lateral dimensions, and shape. This is true for AlN resonators as well as for resonators of other compositions, such as silicon. Through the control of such properties, it is possible to engineer the resonant behavior of the element via its elastic and dielectric characteristics.
Thus, for example, an AlN microresonator RF filter having the known structure shown in a schematic perspective view in FIG. 1 is readily designed to have a passband centered at any frequency in the range 30 kHz to 10 GHz and a Q factor as high as 1500 or even more. One typical center frequency useful for RF communications is 22 MHz.
In the figure, element 10 is an AlN resonant element formed on silicon substrate 20. Wing portions 30, 35 of the resonant element are acoustically isolated from the substrate by etching a trench around them which undercuts the resonant element to form void 40 as best seen in cutaway view 50. Metallization pattern 60 conducts input and output signals between external conductors 70 and upper and lower electrode layers (not shown), which are typically formed adjacent the respective upper and lower faces of element 10. The bottom electrode is typically electrically isolated from the silicon substrate by a silicon oxide layer.
Devices such as the RF filter of FIG. 1 are generally fabricated using well-known wafer-scale integrated circuit (IC) microfabrication techniques such as CMOS techniques. Although the design specifications are generally directed to particular desired resonant frequencies, small process variations (such as variations in film and/or metal thickness across the water) are likely to produce variations in the resonant frequency from lot to lot and even across a single substrate wafer. Those individual devices that best match the desired frequency can of course be selected from a large lot, but such a practice generally leads to low yields and high unit costs. Hence, there remains a need for methods of finely tuning the fabricated devices so that yields of devices that conform to specifications can be improved, and so that particular tuning requirements can be met with high reliability.
AlN resonant elements can also be formed using an interdigitated electrode structure (referred to as width-extensional resonators). These resonant elements are typically used for 500-3000 MHz type devices.